1. Field of the Invention
The present invention relates to an apparatus for establishing a common signal channel, and more particularly to an apparatus for establishing a common signal channel for transmitting various control signals, between exchanges independently of a number of speech channels, the common signal channel being shared by the speech channels.
2. Description of the Related Art
Heretofore, it has been customary to establish a common signal channel for a No. 7 common channel signaling system in the broadband ISDN (B-ISDN) system by designating a certain channel among physical channels between exchanges with a VPI (Virtual Path Identifier) and a VCI (Virtual Channel Identifier) and producing settings in a routing table of an ATM switch thereby to connect the designated channel and a level-2 terminating device to each other.
Specifically, when a serviceman is to establish a common signal channel, the serviceman first determines a suitable physical channel to be used as the common signal channel, and a VPI and a VCI. Based on the information representing the determined physical channel, the VPI, and the VCI, the serviceman enters a command for establishing the common signal channel into a processor in an exchange. In response to the command, the processor initializes a level-2 terminating device and a level-1 terminating device, and sets the common signal channel to a desired channel capacity. Then, the processor produces settings in a routing table of an ATM switch for thereby connecting the level-2 terminating device and the level-1 terminating device to each other.
Since a common signal channel is established when the serviceman enters a command into the processor, a channel capacity to which the common signal channel is set is fixed and not variable. Therefore, when the traffic volume in the common signal channel increases, the common signal channel may possibly suffer congestion even if the traffic capacity between two exchanges or stations interconnected by the common signal channel is sufficiently large, with the result that some of control signals transmitted over the common signal channel may be lost. When such congestion occurs in the common signal channel, the quality of services available between such two exchanges or stations is lowered. The quality of services between the exchanges or stations is also lowered when the common signal channel is subject to a fault. Conversely, when the traffic volume in the common signal channel decreases, the traffic intensity or occupancy of the common signal channel is reduced, leaving an empty traffic volume unused in the common signal channel. However, such an empty traffic volume is not available for communication through the speech channels because the common signal channel is set to the fixed channel capacity.